Relationship Between
Surface Ozone Concentrations and Other Pollutants
(Continued)

Co-occurrence of Ozone and Daily PM2.5

Using 2001 data from the EPA AQS database,
the daily co-occurrence PM2.5 and O3 over a 24-h period was characterized.
There were 362 sites where PM2.5 and O3 monitors were co-located.
As described in the Introduction, a daily co-occurrence of O3
and PM2.5 was subjectively defined as when an hourly average
O3 concentration greater than or equal to 0.05 ppm and a PM2.5
24-h concentration equal to or greater than 40 ug/m3 occurred
over the same 24-h period. Figure 5 illustrates the daily co-occurrence
patterns observed. Using 2001 data from the AQS database, the
daily co-occurrence of PM2.5 and O3 was infrequent.

Figure 5. The co-occurrence pattern for
ozone and PM2.5 using 2001 data from AQS.